We propose to study an apparent dissociation phenomenon of the staphylococcal penicillinase plasmid that has been observed in strains from clinical sources. The homologies of chromosomal DNA from various species and strains of the genus Staphylococcus will be investigated by DNA-DNA hybridization techniques, as well as homologies between DNA from extrachromosomal genetic elements such as plasmids and bacteriophages.